1. Field of the Invention
The invention relates to a slurry hydrocarbon synthesis process which includes isomerization in an external lift reactor loop. More particularly the invention relates to a slurry Fischer-Tropsch type of hydrocarbon synthesis process, in which the synthesized hydrocarbon slurry liquid in the synthesis reactor is circulated through an external lift reactor, in which it reacts with hydrogen in a the presence of a monolithic hydroisomerization catalyst, to hydroisomerize the liquid and reduce its pour point. The hydroisomerized liquid is then passed back into the synthesis reactor.
2. Background of the Invention
The slurry Fischer-Tropsch hydrocarbon synthesis process is now well known and documented, both in patents and in the technical literature. This process comprises passing a synthesis gas, which comprises a mixture of H2 and CO, up into a hot reactive slurry comprising synthesized hydrocarbons which are liquid at the synthesis reaction conditions and in which is dispersed a particulate Fischer-Tropsch type of catalyst. The H2 and CO react in the presence of the catalyst and form hydrocarbons. The hydrocarbon liquid is continuously or intermittently withdrawn from the reactor and pipelined to one or more downstream upgrading operations. The upgraded products may include, for example, a syncrude, various fuels and lubricating oil fractions and wax. The downstream upgrading includes fractionation and conversion operations, typically comprising hydroisomerization, in which a portion of the molecular structure of at least some the hydrocarbon molecules is changed. It would be an improvement if the synthesized hydrocarbon slurry liquid could be hydroisomerized to reduce its pour and melt points, which make it more transportable by pipeline, before it is pipelined to downstream operations.
The invention relates to a slurry Fischer-Tropsch type of hydrocarbon synthesis process, in which a portion of the synthesized hydrocarbon slurry liquid is passed out of the synthesis reactor and into one or more external lift reactors, in which it reacts with hydrogen in the presence of a hydroisomerization catalyst, and preferably a monolithic hydroisomerization catalyst, to hydroisomerize the liquid, which is then passed back into the three-phase slurry (main slurry body) in the reactor. The slurry liquid, which comprises synthesized hydrocarbons that are liquid at the synthesis reaction conditions, comprises mostly normal paraffins and the hydroisomerization reduces its pour and melt points, thereby making it more pumpable and pipelinable. The external lift reactor may comprise a simple vertical, hollow fluid conduit or tube. In operation, hot slurry from the main slurry body, is contacted with means for removing gas bubbles, and preferably both gas bubbles and particulate solids from the slurry liquid which, along with a hydrogen treat gas, is then passed out of the synthesis reactor and up into the one or more lift reactors. The hydroisomerizing catalyst is located in the interior of the lift reactor and comprises the hydroisomerizafion zone. Thus, synthesized hydrocarbon liquid is passed out of the synthesis reactor, up into and through the interior of the lift reactor and back into the synthesis reactor. The hydrogen or hydrogen treat gas injected up into the lift reaction zone, acts as a lift gas to provide circulation of the slurry liquid between the synthesis reactor and the external tube. The slurry circulation up into and out of the lift reactor is achieved by the lifting action of the hydrogen treat gas and, therefore, the external hydroisomerization reactor may be regarded as a form of lift tube or gas lift reactor. The lift reactor is in fluid communication with the main slurry body inside the synthesis reactor, by means of upper and lower conduit portions opening into respective upper and lower portions of the reactor. This enables hydroisomerization of the slurry liquid (i) in an external reaction loop which depends from, and which may therefore be considered as part of the synthesis reactor and (ii) while the synthesis reactor is producing hydrocarbons, but without interfering with the synthesis reaction. The concentration of hydroisomerized hydrocarbon liquid in the synthesis reactor continues to increase until equilibrium conditions are reached. When the synthesis reaches equilibrium, it is possible for the slurry liquid in it to comprise mostly hydroisomerized hydrocarbons of reduced pour point. In some cases, no further hydroisomerization of the liquid hydrocarbon product withdrawn from the synthesis reactor will be necessary. Thus, the process of the invention will reduce and in some cases even eliminate the need for a separate, stand-alone hydroisomerization reactor and associated equipment, downstream of the synthesis reactor. If a downstream hydroisomerization reactor is needed, it will be smaller than it would be if the synthesized hydrocarbon liquid passed into it was not at least partially hydroisomerized. While all of the hydroisomerized hydrocarbon liquid is typically returned back into the main slurry body with which it mixes, in some embodiments a portion of the hydroisomerized liquid will be passed from the lift tube reactor directly to downstream operations.
The use of one or more external hydroisomerization loops associated with the synthesis reactor permits the hydroisomerization temperature to be different (e.g., higher) from that in the synthesis reaction zone. A higher hydroisomerization temperature enables the use of a less expensive, non-noble metal hydroisomerization catalyst. The gas bubble and preferably the slurry gas bubble and solids removal means is preferably located within the main slurry body and may comprise the same or separate means. While various filtration means may be used to separate the slurry liquid from at least a portion of the catalyst particles before the slurry liquid is passed up into the hydroisomerization zone, in the practice of the invention the use of filtration means may be avoided by using known slurry solids reducing means that do not employ filtration. Gas bubble and solids removing means suitable for use with the present invention are known and disclosed in, for example, U.S. Pat. Nos. 5,866,621 and 5,962,537, the disclosures of which are incorporated herein by reference. Simple gas bubble removing means are disclosed in U.S. Pat. Nos. 5,382,748; 5,811,468 and 5,817,702, the disclosures of which are also incorporated herein by reference. Removing gas bubbles from the slurry also densifies it and, if properly used in association with feeding the densified slurry liquid into the hydroisomerization zone (e.g., via a downcomer in the slurry body in the synthesis reactor), will provide a density-difference hydraulic head to assist circulation of the slurry liquid up through the lift reactor. Removing gas bubbles from the slurry prior to hydroisomerization also reduces the CO and water vapor content of the flowing fluid, which could otherwise react with the hydroisomerization hydrogen and also adversely effect the hydroisomerization catalyst. A monolithic hydroisomerization catalyst having a minimal solid cross-sectional area perpendicular to the flow direction of the fluid, minimizes the pressure drop of the fluid flowing up and across the catalyst surface. Removing catalyst and other solid particles, such as inert heat transfer particles, from the slurry upstream of the hydroisomerization zone, reduces scouring of the monolithic catalyst, plugging of the hydroisomerization reaction zone in the tube and also reduces the liquid phase viscosity.
In a broad sense, the process of the invention comprises a slurry Fischer-Tropsch hydrocarbon synthesis process, in which a portion of the hydrocarbon slurry liquid is removed from the slurry body in the synthesis reactor, reduced in gas bubble content and passed up into and through a hydroisomerization zone in a lift reactor external of, and in fluid communication with, the synthesis reactor, in which it reacts with hydrogen in the presence of a hydroisomerization catalyst, at reaction conditions effective to hydroisomerize the hydrocarbon liquid and reduce its pour point, with at least a portion of the hydroisomerized liquid passed back into the main slurry body in the synthesis reactor. Preferably, both gas bubbles and particulate solids are removed from the slurry liquid before it contacts the hydroisomerization catalyst. In a still further embodiment, the invention comprises the steps of:
(i) passing a synthesis gas comprising a mixture of H2 and CO into a main slurry body comprising a three-phase slurry in a slurry Fischer-Tropsch hydrocarbon synthesis reactor, in which the slurry comprises gas bubbles and a particulate hydrocarbon synthesis catalyst in a slurry hydrocarbon liquid, at reaction conditions effective for said H2 and CO to react in the presence of the catalyst and form hydrocarbons, a portion of which are liquid at the reaction conditions and comprise the slurry liquid;
(ii) withdrawing a portion of slurry from the main slurry body;
(iii) contacting the withdrawn slurry with means for removing gas bubbles, to form a slurry hydrocarbon liquid reduced in gas bubbles;
(iv) passing hydrogen and the liquid formed in (iii) into a hydroisomerizing zone in a lift reactor external of, in fluid communication with and depending from, the synthesis reactor, in which they contact a hydroisomerization catalyst and preferably a monolithic hydroisomerization catalyst;
(v) reacting the hydrogen and liquid in the presence of the hydroisomerizing catalyst to hydroisomerize at least a portion of the liquid to form a liquid of reduced pour point, and
(vi) passing all or a portion of the hydroisomerized liquid back into the synthesis reactor.